1. Field of the Invention
The present invention relates to a particle size distribution measuring apparatus, and more particularly to an apparatus for measuring the size distribution of particles by making use of a centrifugal force field.
2. Prior Art
If a suspension is made to revolve, it is put in a centrifugal force field produced by the revolution of the suspension. Therefore, all the solid particles suspended in the suspension receive their respective forces, and begin to move in the suspension. The movements of these particles can be used for measuring the size distribution of the particles. In this case, the resultant force acting on each of the particles is the sum of a centrifugal force and a "buoyancy", because the particles are "immersed" in a liquid medium which, together with the particles, constitutes the suspension. The buoyancy in a centrifugal force field is "centripetal" and depends not only on the density of the medium but also on the position in the suspension, being different from an ordinary buoyancy in the gravitational field. A conventional apparatus for measuring the size distribution of particles by making use of a centrifugal force field commonly comprises a sample vessel, a mechanism for making the sample vessel revolve at a high speed and a particle concentration detector. The sample vessel is to hold a suspension containing test particles whose size distribution is to be measured. The suspension is prepared by making a suitable liquid medium suspend the test particles. With the sample vessel containing the test particles made to revolve at a high speed, the particles begin to move in the suspension owing to the centrifugal force field produced by the revolution. The movements of the particles cause the local particle concentrations (which were uniform throughout the suspension initially) to vary with the passage of time. The moving speeds of the particles are determined from the centrifugal forces and the centripetal buoyancies, both acting on the particles, and from the viscosity of the medium. In addition the centrifugal forces and the buoyancies depend on the sizes and positions of the particles. Therefore, the time-dependent variation of the particle concentration at a position in the suspension gives information about the size distribution of the particles. The particle concentration is detected by said particle concentration detector in the apparatus.
In such a method of measuring the size distribution of particles, it should be noticed that the observed particle concentration is influenced also by a factor not related to the particle sizes. Whether the resultant forces acting on the particles are centrifugal or centripetal, the movements of the particles in a centrifugal force field are not parallel to one another but radial. Therefore, the particle concentration and its time-dependent variation to be observed depend also on the position of concentration detection. The inventors of the present invention have already proposed, in Japanese Patent Application No. 56-214707, a measure to correct the error arising the measured particle size concentration owing to non-parallel centrifugal movements of the particles. However, correction is also necessary to correct for non-parallel centrifugal movements expected when the density of the particles is smaller than that of the liquid medium of the suspension.
By the way, a buoyancy acting on a particle in a suspension put in a centrifugal force field is proportional to the centrifugal force acting on the particle, but it is directed opposite to the centrifugal force. In other words, the buoyancy in a centrifugal force field is centripetal. Therefore, if the centrifugal forces acting on the suspended particles are larger than the buoyancies acting on the same, the resultant forces are still centrifugal, so the movements of the particles are directed outward from the center of revolution of the suspension. On the contrary, if the buoyancies overwhelm the centrifugal forces, the particles receive centripetal forces and move toward the center of revolution of the suspension. As is understood from the previous description about the error arising from the non-parallel movements of the particles, the centrifugal movements of the particles cause the seeming particle concentration to be observed smaller, while the centripetal movements cause the particle concentration to be observed larger. Such being the case, the measure to correct the error due to the non-parallel movements of particles is necessarily designed so as to be applicable to both the two directions of the particle movements.